In accordance with the recent demand for higher density of magnetic recording media, attention is paid on perpendicular magnetic recording media capable of high density recording while avoiding any influence of demagnetizing field in the magnetic layer in-plane direction.
Evaporated or sputtered films of Co--Cr series alloy are known as the magnetic layer for perpendicular magnetic recording media. In magnetic layers using a Co--Cr series alloy containing at least a certain amount of Cr, columnar crystal grains of the hexagonal system grow in a perpendicular direction with axis c aligned with the perpendicular direction.
Perpendicular magnetic recording media using a perpendicular magnetizable film of Co--Cr series alloy as the magnetic layer generally have a Permalloy film beneath the magnetic layer. The Permalloy film is effective for allowing magnetic flux to escape from each columnar crystal grain of the magnetic layer, thereby preventing formation of a closed loop within each columnar crystal grain due to demagnetizing field. If a closed loop were formed within the respective columnar crystal grains, no magnetic flux would extend beyond the magnetic layer surface, resulting in a substantial loss of output. The provision of a Permalloy film below the magnetic layer causes the magnetic flux of one columnar crystal grain to enter other columnar crystal grains, thus allowing for leakage of magnetic flux beyond the magnetic layer surface for enabling reproduction.
On the contrary, great efforts have been devoted on magnetic recording media using a ferromagnetic metal thin film of Cr-free Co base alloy, for example, Co base alloy having Ni and other elements added thereto, because of their high saturation magnetic flux density and high coercivity. If such a Co base alloy is perpendicularly grown like the Co--Cr series alloy, the resulting magnetic layer is not magnetizable perpendicularly due to the influence of demagnetizing field in the thickness direction. Thus this magnetic layer does not have an axis of easy magnetization or develop coercivity.
As a consequence, ferromagnetic metal thin film magnetic layers of Co base alloy such as Co--Ni alloy are generally formed by the oblique evaporation technique.
The oblique evaporation technique carries out evaporation by feeding a non-magnetic substrate around the surface of a rotating cylindrical chill drum and irradiating a stationary ferromagnetic metal source with an electron beam or the like. There was proposed a multi-layered structure in which more than one layer of ferromagnetic metal thin film is stacked by such an oblique evaporation technique. Generally in this case, columnar crystal grains of one ferromagnetic metal thin film layer are grown in a select direction intersecting the growth direction of columnar crystal grains of other ferromagnetic metal thin film layers (see Japanese Patent Publication Nos. 26891/1981, 42055/1981, 21254/1988 and 37528/1985 and Japanese Patent Application Kokai Nos. 603/1979, 147010/1979, 94520/1981, 3233/1982, 30228/1982, 13519/1982, 141027/1982, 41028/1982, 141029/1982, 143730/1982, 143731/1982, 147129/1982, 14324/1983, 50628/1983, 76025/1985, 110333/1986, 187122/1986, 10315/1988, 10315/1988, 13117/1988, 14317/1988, 14320/1988, and 39127/1988).
In this regard, the angle between the incident direction of ferromagnetic metal and a normal to the surface of the non-magnetic substrate is designated an incident angle. Evaporation is carried out such that the incident angle gradually decreases from the start to the end of evaporation.
The rate of evaporation is minimum at the start of evaporation when the incident angle is at maximum and rapidly increases as the incident angle increases. As a consequence, the columnar crystal grains in the ferromagnetic metal thin film deposited on a non-magnetic substrate are oriented approximately parallel to the substrate surface where they are adjacent to the substrate, but rapidly raised up as they are spaced apart from the substrate surface, growing in an arcuate manner.
Such a ferromagnetic metal thin film has an axis of easy magnetization whose direction depends on the gradient of columnar crystal grains. Since the maximum incident angle is generally 90.degree., the gradient of columnar crystal grains largely depends on the minimum incident angle.
If the minimum incident angle is reduced, the gradient of columnar crystal grains with respect to a normal to the substrate is also reduced. Differently stated, columnar crystal grains stand up with respect to the substrate and their axis of easy magnetization also stands up with respect to the substrate.
Where a ferromagnetic metal thin film formed by such an oblique evaporation technique is applied to perpendicular magnetic recording media, it is necessary to prevent formation of a closed loop within columnar crystal grains and hence, to dispose a Permalloy film below the magnetic layer.
However, the Permalloy film which is an Fe--Ni alloy is less resistant against corrosion. In addition, local current can flow between the Co base alloy and the Fe base alloy, leaving a problem of corrosion resistance in this sense too. Further, the Permalloy film is a soft magnetic film which is non-magnetizable and contributes to no output improvement.